Geotropic response of cucumber hypocotyls

Exogenous auxin applied to the apical bud of intact cucumberseedlings promoted elongation of the hypocotyl, mostly of theapical zone (zone I) and to a lesser extent, the middle andbasal zones (zones II and III). Geotropic curvature was clearlyobserved 30 min after geostimulation. The curvature began atzone I and moved toward the basal zones with prolonged geostimulation.Decapitation (removal of apex and cotyledons) did not produceany appreciable effect on the geotropic response, but the curvaturewas reduced with the removal of zone I. Auxin application toseedlings without zone I restored their ability to respond togravity. The lower epidermis of horizontally oriented hypocotylsplayed a more important role than the upper in the geotropicresponse. The extensibility of the lower epidermal cell wallwas increased significantly by the geostimulus before the onsetof the curvature response. (Received August 24, 1973; )